"The significant increase in fatal crashes involving marijuana is alarming," Peter Kissinger, President and CEO of the AAA Foundation for Traffic Safety, said in a news release about the organization's findings. "Washington serves as an eye-opening case study for what other states may experience with road safety after legalizing the drug."

Washington state, along with at least five other states, have set what are known as per se limits—limits on how much active THC, the main psychoactive compound in cannabis plants, a driver can have in his or her system, the AAA foundation says. But the safety foundation described such limits as "arbitrary" and "unsupported by science," and has called on law enforcement to instead test for recent marijuana use and determine if there's physiological and behavioral signs of impairment in a driver.

To come to its conclusion, the AAA foundation analyzed data related to almost 5,500 drivers who were arrested for impaired driving and found to have some form of active THC in their blood. The organization said there was no correlation between specific levels of the substance in the systems and their poor driving. Some drivers with low levels of active THC could be considered unsafe at the wheel after assessment, but others who had consumed higher levels of the drug could appear capable of driving safely, the study concluded.

The drug is legal for medicinal use in 20 states, and up to 11 more states will vote to legalize it for for either medicinal or recreational use, or both, this year, the Associated Press reports. Marijuana remains illegal at the federal level.